1. Field of the Invention
The present invention relates to aeration systems and, more particularly, systems for aerating effluent in wastewater treatment plants, or the like.
2. Description of the Prior Art
In wastewater treatment plants, it is known to aerate effluent or sludge as part of the wastewater purification process commonly known as the "activated sludge" process. This process typically includes a biological treatment system consisting of a contained volume of wastewater with an aerobic microorganism colony which thrives on biodegradation of the wastewater in an aerobic environment. In order to maintain the requisite aerobic environment, supplemental air must be introduced into the system to promote the dissolution of gaseous oxygen into the liquid phase. The introduction of supplemental air is commonly achieved through the use of either mechanical surface aerators or submerged diffused aerators.
The rate of dissolution of gaseous oxygen into the liquid phase is directly proportional to (1) the surface area of the gas bubbles produced by these aerators, wherein the oxygen molecule travels across this "boundary" from the gaseous phase within the bubble to the liquid phase without the bubble; and (2) the residence time, i.e., the length of time the gas bubble is exposed to the liquid. For example, several hundred smaller gas bubbles contain more surface area than one large bubble containing the same total volume of air, and hence, the smaller bubbles provide more efficient oxygen transfer. Due to friction losses, however, the energy required to produce fine bubbles increases as the size of the diffuser orifices decreases. As a result, fine bubble diffusers which provide more efficient oxygen transfer, have high operating power costs, while larger bubble jet diffusers, i.e., "coarse bubble jet diffusers", which produce a larger bubble size with less oxygen transfer efficiency, have a lower operating power cost.
The coarse bubble diffuser offers the advantage of higher exit velocity which provides greater mixing activity within the aeration tank, resulting in increased residence time of the air in solution. This, in turn, provides an increase in the total oxygen transferred while increasing the oxygen transfer efficiency closer to that of a fine bubble system. Complete mixing of the microorganism colony within the aeration tank system is critical to ensure adequate waste treatment within the aeration tank. Inadequate mixing can result in portions of the microorganism colony to settle to the tank bottom thereby limiting exposure of the microorganisms to the wastewater.
It is an object of the present invention to provide a diffused aeration system with improved overall oxygen transfer efficiency with little or no concomitant increase in operating power cost.
It is a further object of the present invention to provide a diffused aeration system having improved oxygen transfer efficiency while providing improved mixing capacity.